The traditional rapid sand filter has been a reliable performer in potable water treatment and the mechanics of the operation and performance of such a filter have remained largely unchanged over the years. The filter is a straining device comprising a bottom underdrain collection system equipped with slotted strainers or the like which holds a layer of filter sand. Above the sand layer is a layer (or layers) of hard coal media which is coarser. A wash water trough is located above the media layers and is used to direct unfiltered water into the filter as well as to channel backwash rinse water to a waste outlet. A backwashing cycle is required when the filter media has substantial head loss during operation, sometimes occurring several times a day due to particle buildup in the filter. Backwashing fluidizes the media, rinses out the particles from the interstitial voids and reclassifies the layers of media
A problem associated with backwashing is the non-uniform or uneven water distribution which occurs because of the momentum of the water discharging from the perforated header or channel. Water at high velocity across an orifice will not be discharged through the orifice as readily as when flowing at lower velocity. Backwash discharge from the underdrain will be greatest in those portions of the underdrain furthest away from the backwash water inlet. Such unequal flows cause undesirable channelling in the media which reduces the efficiency of the backwash operation and results in filtering problems.
A further problem with many types of filter is the requirement for a support gravel layer immediately above the underdrain upon which the sand layer rests. This requirement results in a deeper overall bed and increased material requirements for the filter installation. The need for a support gravel layer also restricts the available depth for the sand and coal layers which reduces the filtration effectiveness.
The above-identified problems are discussed in various patents. Such prior art patents include U.S. Pat. No. 3,956,134 (Sturgill) dated May 1976 and entitled UNDERDRAIN FOR WATER FILTRATION SYSTEM; U.S. Pat. No. 4,214,992 (Sasano et al) dated August 1978 and entitled WATER COLLECTING AND DISTRIBUTING APPARATUS DISPOSED IN A LOWER PORTION OF HIGH SPEED FILTER BASIN; U.S. Pat. No. 4,331,542 (Emrie) dated August 1980 and entitled UNDERDRAIN UNIT WITH AIR/WATER BACKWASH FOR GRANULAR FILTRATION SYSTEM; U.S. Pat. No. 4,923,606 (Gresh et al) dated February 1988 and entitled PLASTIC JACKETED FILTER UNDERDRAIN BLOCK; U.S. Pat. No. 4,995,990 (Weston) dated April 1989 and entitled AIR AND WATER DISTRIBUTION CONDUIT; U.S. Pat. No. 5,068,034 (Walter) dated May 1990 and entitled PURIFICATION UNDERDRAIN WITH MEANS COMPENSATE FOR FLOW AND PRESSURE DIFFERENCES BETWEEN LATERALS; U.S. Pat. No. 5,160,614 (Brown) dated February 1992 and entitled AIR DUCT BLOCK FOR AIR/WATER UNDERDRAIN SYSTEMS IN GRAVITY FILTERS; U.S. Pat. No. 5,149,427 (Brown et al) dated September 1992 and entitled CAP FOR UNDER DRAINS IN GRAVITY FILTERS; U.S. Pat. No. 5,413,710 (Roberts et al) dated May 1995 and entitled LATERAL UNDERDRAIN; and U.S. Pat. No. 5,462,664 (Neuspiel) dated October 1995 and entitled FILTER UNDERDRAIN MODULE AND UNDERDRAIN SYSTEM.
The prior art identified above suffers from other problems, such problems including that the devices disclosed are not readily adaptable for retrofitting to existing filter installations; that the devices are prone to plugging with grit flushed in during the-backwash cycle and are therefore less resistant to structural failure during the large upward hydraulic thrust generated during backwash; that the devices are expensive to purchase and are difficult and expensive to install, some requiring tedious grouting procedures or cumbersome and expensive false bottom structures; that many of the devices are constructed from tile or porous tile or the like and are therefore fragile and subject to breakage during installation while inadequate corrosion resistance is also present; and that some devices require support gravel layering.
Our earlier U.S. Pat. No. 5,019,259 (Hambley) dated May 28, 1991 and entitled FILTER UNDERDRAIN APPARATUS WITH PARTITIONED DISTRIBUTOR CONDUITS, the contents of which are incorporated herein by reference, teaches a filter underdrain apparatus which includes a steel flute or arch forming a plurality of horizontal distributor conduits. The conduits are in juxtaposed, laterally spaced relationship and are constructed to define alternating conduits and troughs of a filter underdrain. This apparatus functions well with or without support gravel and reduces backwash water maldistribution by varying the diameter of the water inlet/outlet orifices arranged along the length of the water conduits. While this apparatus is effective in overcoming many of the problems of the prior art, the effective diameter of each individual inlet/outlet orifice in the flute or arch needed to be calculated and attendant tool changes were required during the manufacturing process to accommodate the variations in orifice size along the flute.